The present invention relates to a control unit for magnetic bearing.
A rolling element rotated as supported by a magnetic bearing in non-contact fashion encounters resonance if an increasing rotational vibration frequency coincides with a natural frequency when, for example, the rolling element is brought from a stationary state into a start-up state increasing a number of rotation. Due to a so-called gyroscopic action, the natural frequency of the rolling element varies with increase or decrease in the number of rotation thereof.
In order to prevent such resonance as mentioned above, a control unit for magnetic bearing, as disclosed in an official gazette of Japanese unexamined patent publication 63(1988)-275814, is arranged such that a voltage-variable notch filter is interposed between a magnetic-bearing output circuit and a power amplifier for power supply to the magnetic bearing. The voltage-variable notch filter is adapted for sequential variation of a central frequency thereof through voltage, which is obtained through F/V conversion of the number of rotation of the rolling element. The central frequency of the notch filter is so controlled as to vary in correspondence with the natural frequency varying with increase or decrease in the number of rotation of the rolling element. As a result, a resonance signal at the same frequency with the natural frequency is eliminated and thus, the resonance is prevented.
The above conventional control unit for magnetic bearing is totally composed of analog circuitry. However, a control unit principally composed of digital signal processing circuitry is becoming predominant recently. One of the features of the control unit principally composed of the digital signal processing circuitry is that all but an input and an output portions are involved in digital processings. If the above voltage-variable notch filter is provided, however, this portion must be provided as an additional analog circuit. This constitutes a problem of complicated circuit configuration.
In view of the foregoing problem encountered in the art, the present invention has an object to provide a control unit for magnetic bearing adapted to prevent the occurrence of resonance by way of digital control.
A control unit for magnetic bearing in accordance with the present invention comprises: number-of-rotation sensing means for sensing a number of rotation of a rolling element supported by a magnetic bearing; sampling frequency decision means for varying a sampling frequency according to variation of the number of rotation sensed by the number-of-rotation sensing means; displacement sensing means for sensing a displacement of the rolling element and outputting a digital displacement signal based on a sensed displacement; control means for outputting a control signal for positional control of the rolling element based on the digital displacement signal outputted from the displacement sensing means, the control means performing a predetermined digital filtering processing at the sampling frequency provided by the sampling frequency decision means thereby varying a cutoff frequency according to the sampling frequency, to output the control signal with a damped frequency band including a resonance frequency associated with a natural frequency varying with the number of rotation of the rolling element and a rotational vibration frequency; and electromagnet control means for controlling an electromagnet of the magnetic bearing based on the control signal outputted from the control means.
In the control unit for magnetic bearing arranged as mentioned above, the sampling frequency decision means varies the sampling frequency according to the variation of the number of rotation. The control means for outputting the control signal for positional control of the rolling element performs the predetermined digital filtering processing at the sampling frequency thereby varying the cutoff frequency according to the variation of the sampling frequency. Thus, the frequency band to be xe2x80x9ccut offxe2x80x9d is varied in correspondence with the resonance frequency associated with the natural frequency corresponding to the number of rotation of the rolling element and the rotational vibration frequency, so that outputted is the control signal with the damped frequency band which includes the resonance frequency. Consequently, the resonance signal is eliminated and thus, the resonance can be prevented.
In the aforementioned control unit, the sampling frequency decision means may decide the sampling frequency according to the data table.
In this case, if the data table is changed, the variation characteristics of the sampling frequency are varied and hence, the variation characteristics of the cutoff frequency are varied, as well. Therefore, even if the characteristics of the natural frequency of the rolling element vary, the resonance signal can be damped by accordingly changing the data table or varying the cutoff frequency. Therefore, if the variation characteristics of the cutoff frequency are selected to meet the condition of use of the rolling element (for instance, the type of tool to be connected in a case where the rolling element is a spindle for use in the machine tool), the resonance can be prevented in any of the various conditions of use of the rolling element.